1. Field of the Invention
This invention relates generally to optical recording medium capable of performing recording/reproduction of information by application of a light beam. More particularly, the invention relates to an optical recording medium having a transparent resin substrate, a transparent adhesive layer, a transparent protective layer and a recording layer, and a method of manufacturing the medium.
2. Description of the Prior Art:
An optical information-recording medium that performs recording/reproduction of information by applicatiOn of a light beam has advantages in that a large storage capacity is available due to its extremely high density of recording. Also, non-contact recording/reproduction of information causes no wear of the medium. Thus, the its practical application of this medium to optical discs or optical cards has been considered.
Optical recording media can be classified into various kinds in terms of materials used for their recording layers. However, among these kinds, an optical recording medium of the type that can be used to record information typically has a basic structure including a recording layer on a substrate. The optical properties of the media changed when a light beam is applied to the substrate. The optical recording medium of this type is generally used by applying a laser beam through the substrate to the recording layer, and detecting the light reflected therefrom so as to reproduce information. In addition, in order to obtain high speed access this optical recording medium is generally provided with a groove on the substrate that guides an optical head.
For the reason above, such transparent resin materials as polymethyl methacrylate, polycarbonate, epoxy and polyolefin, that are superior in optical properties and groove formation readiness, are suitable as substrate materials.
On the other hand, for the recording layer materials, such materials as Te-C, TeOx, and TeCS.sub.2 have already been put into practice in a DRAW (Direct Read After Write) type optical recording medium, i.e., an optical recording medium of a non-erasable type. In a second generation E-DRAW (Erasable DRAW) type, i.e., a rewritable type, optical recording medium, several media are generally known. These include a magneto-optic type recording medium in which the recording layer utilizes a rare earth-transition metal amorphous alloy thin film (hereinafter simply referred to as RE-TM film), a phase change type optical recording medium in which the recording layer is a thin film of chalcogen compound such as InSe that utilized transformation from amorphous state to crystal state, and an optical recording medium in which the recording layer is a thermo-plastic film.
However, of the recording layer materials described above, those having superior adhesive properties with respect to transparent resin materials, which are preferable as substrate materials, are limited to semi-organic group materials such as Te-C, or organic materials such as organic dye. In general, materials such as metals, semiconductors and low-grade oxides have limited adhesive properties with respect to resin materials. As a result, there have been problems in reliability. For example, such layers can readily peel off the substrate when tested with adhesive tape, or peel-off phenomena can occur after an accelerated deterioration test under high temperature with humidification.
On the other hand, an optical disc was disclosed in
Laid Open Patent No. 60-7954, which corresponds to U.S. Pat. No. 4,596,740, in which an organic thin film was used as a transparent adhesive layer between a transparent resin substrate and a recording layer. According to this publication, the optical disc had a structure in which a polymer layer was sandwiched between the transparent resin substrate and the recording layer. This polymer layer had satisfactory adhesive properties with respect to the resin substrate, but could not prevent moisture from penetrating through the resin substrate. Specifically, the resin substrate and the adhesive layer of organic thin film should be matched in terms of thermal constants and mechanical constants, so that the adhesive properties therebetween are satisfactory. However, both the resin substrate and the adhesive layer of organic thin film are permeable to gaseous matter. Thus, the recording layer may be corroded by moisture penetrating through the layers. This has caused problems in reliability. Thus, when metallic group materials are used in the recording layer, such as RE-TM film which is not environment-proof, an additional transparent protective layer consisting of transparent inorganic materials should be sandwiched between the transparent adhesive layer and the recording layer. This transparent protective layer should consist of materials which are so dense as to preclude moisture permeation. Accordingly, thin films of transparent inorganic materials such as Si.sub.3 N.sub.4, AlN, BN, Al.sub.2 O.sub.3 and SiO.sub.2 are used. However, inorganic thin films generally have properties that differ significantly from those of organic thin films in terms of thermal constants (thermal-expansion coefficients) and mechanical constants (Such as Young's modulus and Poisson ratios). Therefore, the transparent protective layer consisting of transparent inorganic material has extremely poor adhesive properties with respect to both a resin substrate and a transparent adhesive layer consisting of only organic thin film. As a result, the transparent protective layer peels off the resin substrate or peels off the transparent adhesive layer, or the transparent protective layer, itself, cracks.
As described above, in conventional optical recording medium, an Organic thin film sandwiched between a transparent resin substrate and a recording layer exhibits satisfactory adhesive properties. However, this medium has a problem in that moisture can permeate the medium.